Latin for “who will guard the guards themselves?”
Generally used to describe a situation in which a person or body having power to supervise or scrutinise the actions of others, is not itself or themselves subject to supervision or scrutiny.
An example, cited by Lord Justice Toulson in the case of R (Guardian News and Media Ltd) v City of Westminster Magistrates’ Court  EWCA Civ 420;  QB 618 at , is the courts themselves:
“Open justice. The words express a principle at the heart of our system of justice and vital to the rule of law. The rule of law is a fine concept but fine words butter no parsnips. How is the rule of law itself to be policed? It is an age old question. Quis custodiet ipsos custodes—who will guard the guards themselves? In a democracy, where power depends on the consent of the people governed, the answer must lie in the transparency of the legal process. Open justice lets in the light and allows the public to scrutinise the workings of the law, for better or for worse. Jeremy Bentham said in a well known passage quoted by Lord Shaw of Dunfermline in Scott v Scott  AC 417, 477: ‘Publicity is the very soul of justice. It is the keenest spur to exertion and the surest of all guards against improbity. It keeps the judge himself while trying under trial.'”
Another example (in some ways related) is the power of the press to hold public figures to account. Who holds the press to account? It can’t be the government, because that would threaten the potential for the press to hold it to account. It can’t be the courts, because open justice depends on scrutiny by the press acting (in theory) as the “eyes and ears of the public”. Attempts to create a credible regulatory regime, following the recommendations of the Leveson Inquiry, have not, as yet, proved successful. Self-regulation is manifestly not credible, at any rate in jurisprudential terms: nemo iudex in causa sua. And so the press remains largely unregulated.